The invention deals generally with automated populating of circuit boards with electrical components. More particularly, the invention is directed towards the placement of surface mounted components (SMC's) where a viscus fluid must first be placed on the board as part of the placement process.
Typically during the placement process, circuit boards are conveyed to each of a sequence of stations of an automated machine or string of automated machines, with a different part of the process being performed at each station. At the fluid dispensing station, the board is repositionable in X and Y relative to a fluid dispenser such as that described in U.S. Pat. No. 4,572,103 in order to place a dot, or a multiplicity of dots, of fluid on the board at each location on which a component subsequently is to be placed.
The size of each dot, as well as the quantity of fluid in each dot, needs to be consistent for each of a given type and/or size of components in order to insure in the case of a glue-like fluid that the component placed on the dot will be retained or in the case of the application of a fluid flux that the subsequent soldering process will be enhanced.
Also, the attendant of the fluid dispensing station needs to know how much fluid is in the syringe supplying fluid to the dispensing nozzle, as dispensing progress, in order to know when to replace an empty or nearly empty syringe with a full one so as to result in a minimum amount of downtime for any given process run.
Prior art mounting of the syringe of the dispensing nozzle involves manipulation and connection of a cover and integrally attached air hose to the distal (plunger) end of the syringe and subsequent threading of the "Luer lock", at the proximal (dispensing outlet) end of the syringe, onto the nozzle of the fluid dispensing station. These operations require considerable operator training, dexterity and results in unwanted loss of production during the machine downtime to perform these functions. Also the Luer lock is fragile and "strips out" if not threaded properly resulting in pressurized adhesive squirting out of the end of the syringe with considerable downtime for cleanup. In addition to requiring special attention and care in securing the syringe to the nozzle, difficulties in removing prior art covers have resulted from the bayonet-type connection, as well as from size variations in the connection portions of the distal ends of the syringes.
Thus, it is an object of the invention to overcome prior art deficiencies which cause difficulties in insuring a reliable connection of the syringe to the nozzle and in easily connection and disconnecting the cover of the distal end of the syringe.
In order to know when the supply of fluid is depleted sufficiently to need replacement, the prior art has involved (i) an attendant manually checking the level of fluid in the syringe, (ii) use of a capacitive sensor, or (iii) use of a n inductive sensor positioned on the outside of the syringe with a metal disk situated on the inside of the syringe and moveable to notify the sensor when the fluid supply is low.
Manually checking the level of fluid requires that the attendant stop the machine. Thus, checking the level during a run and/or running out of fluid in the middle of a run, rather than replacing syringes as needed between runs, can and does result in unnecessary and inefficient downtime in the automated processing of SMC's. Also, if the attendant does not have a constant indication of the amount of adhesive remaining in a syringe, there is wasted processing time between recognizing an empty syringe and replacing it with a refill.
Use of a capacitive sensor presents measurement inaccuracies due to characteristic changes in the sensor with temperature and/or humidity changes.
A disadvantage with inductive sensor/metal disc devices of the prior art is the additional requirement for moving the disc from an empty syringe to its full replacement, which can and has resulted in loss of the disc. Also, these prior art metal discs have gotten stuck in residual adhesive remaining on the inside wall of a syringe as adhesive is dispelled. This presents additional difficulties in removing the disc from the empty syringe and requiring cleaning of the disc or the expense of replacement with a new disc before use in the replacement syringe.
Thus, it is an object of the invention to provide for constant monitoring of the quantity of fluid which is available at the dot dispensing station, so that supply syringes can be replaced in a timely and efficient manner.
Additionally, it is an object of the invention to provide a reliable apparatus of monitoring the fluid available at the station for dispensing without requiring the attendant to retrieve, reposition and/or clean loose parts of the sensor mechanism.
Prior art devices apply pressurized air to the distal end of the syringe as the only motive force for moving the plunger and, in turn, the fluid within the syringe. Typically, an air hose connects the source of pressurized air to a removable cover for the distal end of the syringe. If the plunger gets "hung up" in the syringe, as sometimes occurs, it can become tilted or deformed so that the seal between the plunger and the inner surface of the syringe fails and pressurized air bypasses the plunger and aerates the fluid.
As a result of such aeration, the amount of fluid in the dots being dispensed becomes unreliable and, since air can take the place of at least some of the fluid of a dot being dispensed, cia result in unreliable attachment of the surface mounted electronic components (SMC's) to the circuit boards.
Accordingly, it is an object of the invention to minimize such "hang-ups" that cause breakage of the seal between plunger and syringe and the resulting degradation of the desired fluid dot by pressurized air.
The reader's attention is directed to the distinction which is made, throughout this disclosure, between the "piston" of an air cylinder and the "plunger" of a syringe-type adhesive supply.